Elasticsearch can usually be upgraded using a Rolling upgrade
process so upgrading does not interrupt service. However, you might
need to Reindex to upgrade indices created in older versions.
Upgrades across major versions prior to 6.0 require a Full cluster restart.

When upgrading to a new version of Elasticsearch, you need to upgrade each
of the products in your Elastic Stack. The steps you need to take to upgrade
differ depending on which products you are using. Want a list that’s tailored
to your stack? Try out our Interactive Upgrade Guide. For
more information about upgrading your stack, see Upgrading the
Elastic Stack.

[a] You must delete or reindex any indices created in 2.x before upgrading.

Elasticsearch can read indices created in the previous major version.
Older indices must be reindexed or deleted. Elasticsearch 6.x
can use indices created in Elasticsearch 5.x, but not those created in
Elasticsearch 2.x or before. Elasticsearch 5.x can use indices created in
Elasticsearch 2.x, but not those created in 1.x or before.

This also applies to indices backed up with snapshot and restore. If an index was originally created in 2.x, it cannot be
restored to a 6.x cluster even if the snapshot was created by a 5.x cluster.

Elasticsearch nodes will fail to start if incompatible indices are present.